Making Web Services Accessible from Script

In order for a Web service to be accessed from script, it must be an .asmx Web service whose Web service class is qualified with the ScriptServiceAttribute attribute. Individual methods to be called from script must be qualified with the WebMethodAttribute attribute.

The following example shows these attributes in Web service code.

 

[ScriptService]
public class SimpleWebService : System.Web.Services.WebService
{

[WebMethod]
public string EchoInput(String input)
{
// Method code goes here.
}
}

To enable Web service calls from script, 

you must register the ScriptHandlerFactory HTTP handler in the application's

 Web.config file. The handler processes calls made from script to .asmx 

Web services. 

The following example shows the Web.config element for adding the handler. 

 

type="System.Web.Script.Services.ScriptHandlerFactory"
validate="false"/>



For Web service calls that are not issued from ASP.NET AJAX script, 
the ScriptHandlerFactory handler delegates the call to the default handler, 
which uses SOAP instead of JSON format. 
The delegation is performed automatically and you do not have to take any 
action unless you want to disable the use of the SOAP protocol for the Web 
services. In that case, you must enter the following configuration setting 
in the Web.config file.

Using Web Services in ASP.NET AJAX

This topic describes how to access Web services from client script in AJAX-enabled ASP.NET Web pages. The services can be either custom services that you create, or built-in application services. The application services are provided as part of ASP.NET AJAX, and include authentication, roles, and profile services.

Custom Web services can be in the form of .ASP.NET Web services (.asmx services), or Windows Communication Foundation (WCF) services (.svc services).
 
ASP.NET enables you to create Web services that can be accessed from client script in Web pages. The pages communicate with the server through a Web service communication layer that uses AJAX technology to make Web service calls. Data is exchanged asynchronously between client and server, typically in JSON format.
 

Client-Server Communication for AJAX Clients

In AJAX-enabled Web pages, the browser makes an initial request to the server for the page, and then makes subsequent asynchronous requests to Web services for data. The client communication elements are in the form of downloaded proxy classes from the server and the core client-script library. The server communication elements are handlers and custom services. The following illustration shows the elements that are involved in the communication between the client and the server.

AJAX Client Architecture

Browsers call Web service methods by using proxy classes. A proxy class is a script that is automatically generated by the server and downloaded to the browser at page-load time. The proxy class provides a client object that represents the exposed methods of a Web service.

To call a Web service method, client script calls the corresponding methods of the proxy class. The calls are made asynchronously, through the XMLHTTP object.

The Web service communication layer contains the library script types that enable the proxy classes to make service calls. For more information, see the classes that are contained in the Sys.Net namespace.

The code in the proxy classes and in the core Web service communication layer hides the complexity of XMLHTTP and the differences between browsers. This simplifies the client script that is required to call the Web service.

There are two approaches to making a Web service request:

Call Web services by using the HTTP POST verb. A POST request has a body that contains the data that the browser sends to the server. It does not have a size limitation. Therefore, you can use a POST request when the size of the data exceeds the intrinsic size limitation for a GET request. The client serializes the request into JSON format and sends it as POST data to the server. The server deserializes the JSON data into .NET Framework types and makes the actual Web service call. During the response, the server serializes the return values and passes them back to the client, which deserializes them into JavaScript objects for processing.

Call Web services by using the HTTP GET verb. This resembles the functionality of a POST request, with the following differences:

The client uses a query string to send the parameters to the server.

A GET request can call only a Web service method that is configured by using the ScriptMethodAttribute attribute.

Data size is limited to the URL length allowed by the browser.

 

AJAX Client Architecture

 

Elements of the client architecture include the Web service communication layer in the core library and the downloaded proxy classes for the services that are used on the page. Individual elements shown in the figure are as follows:

Custom Service Proxy Classes. These consist of client script that is automatically generated by the server and downloaded to the browser. The proxy classes provide an object for each WCF or ASMX service that is used in the page. (That is, they provide an object for each item in the ServiceReferences element of the ScriptManager control in the page.) Calling a proxy method in client script creates an asynchronous request to the corresponding Web service method on the server.

Authentication Proxy Class. The AuthenticationService proxy class is generated by the server authentication application service. It enables the user to log on or log out through JavaScript in the browser without making a round trip to the server.

Role Proxy Class. The RoleService proxy class is generated by the server roles application service. It enables you to group users and treat each group as a unit through JavaScript without making round trips to the server. This can be useful to enable or deny access to resources on the server.

Profile Proxy Class. The ProfileService class is generated by the server profile application service. It makes the current user's profile information available to the client through JavaScript without making round trips to the server.

Page Methods Proxy Class. This provides the scripting infrastructure for client script to call static methods in an ASP.NET page as if they were Web service methods. For more information, see Calling Web Services from Client Script.

Web Service Communication Layer. This is the library that contains the client script types. These types enable the browser (client) to communicate with the services on the server. They also shield client applications from the complexities of establishing and maintaining the asynchronous communication between client and server. They encapsulate the browser XMLHTTP object that provides the asynchronous capability, and enable client applications to be browser independent. The following are the main elements of the Web service communication layer:

WebRequest . This provides client script functionality to make a Web request. For more information, see the WebRequest class.

WebRequestManager . This manages the flow of the Web requests issued by the WebRequest object to the associated executor object. For more information, see the WebRequestManager class.

XmlHttpExecutor . This makes asynchronous network requests by using the browser's XMLHTTP support. For more information, see the XmlHttpExecutor class.

JSON Serialization. This serializes JavaScript objects into JSON format. Deserialization is available by using the JavaScript eval function. For more information, see the JavaScriptSerializer class.

The default serialization format is JSON, but individual methods in Web services and in ASP.NET Web pages can return alternative formats such as XML. The serialization format of a method can be specified with attributes. For example, for an ASMX service, you can set the ScriptMethodAttribute attribute to cause a Web service method to return XML data, as shown in the following example:
 
[ScriptMethod(ResponseFormat.Xml)]

AJAX Server Architecture

Elements of the server architecture include the Web service communication layer with a HTTP handler and serialization classes, custom services, page methods, and application services. Individual elements shown in the figure are as follows:

Custom Web Services. These provide the service functionality that you implement and return the appropriate response to the client. Custom Web services can be ASP.NET or WCF services. The Web service communication layer automatically generates client-script proxy classes that can be called asynchronously from client script.

Page Methods. This component enables a method in an ASP.NET page to be called as if it were a Web service method. Page methods must be defined in the page that is performing the page-method call.

Authentication Service. The authentication service generates an authentication proxy class that enables the user to log on or log out through client JavaScript. This application service is always available and you do not have to instantiate it. For more information, see Using Forms Authentication with ASP.NET AJAX.

Role Service. The role service generates a role proxy class that enables client JavaScript to access roles information for the currently authenticated user. This application service is always available and you do not have to instantiate it. For more information, see Using Roles Information with ASP.NET AJAX.

Profile Service. The profile service generates a profile proxy class that enables client JavaScript to get and set profile properties for the user that is associated with the current request. This application service is always available and you do not have to instantiate it. For more information, see Using Profile Information with ASP.NET AJAX.

JSON Serialization. The server JSON serialization component enables customizable serialization and deserialization of common .NET Framework types to and from JSON format. For more information, see JavaScriptSerializer.

XML Serialization. The Web service communication layer supports XML serialization for SOAP requests to Web services and for returning XML types from a JSON request to a Web service.

reference: http://msdn.microsoft.com/en-us/library/bb515101.aspx

 

Authentication for Web Services (using SOAP headers)

http://www.codeproject.com/KB/cpp/authforwebservices.aspx

.Net web services interview questions and answers

What is .Net Web Service?
Web service is the way to publish application's function on web that can be accessible to the rest of the world. Web services are the components that can be used by other applications..............

Define SOAP, WSDL, and UDDI.
SOAP, Simple Object Access Protocol is a communication protocol, a way to structure data before transmitting it, is based on XML standard. It is developed to allow communication between applications of different platforms and programming languages via internet..................

Explain the protocols a .Net Web Service uses.
In .Net, a web service is bind with three different protocols such as HTTP/POST, HTTP/GET, and SOAP. This allows client with three options to choose for communication..............

Explain how to document web services.
ASP.NET web services are considered as self documenting as they provides all information about what methods are available and what parameters they require using XML based standard called WSDL.............

Explain when do we required ASP.NET web services.
ASP.NET web services are the great way to expose your middle tier components via internet. These components offer no issue communicating across firewalls as they use SOAP as transport protocols..............

Why do we need Web Services?
We have a number of heterogeneous technologies available on internet. The demand for reusable components across platforms and programming languages are high....................

Explain in brief different distributed technologies.
The need of distributed technologies arises with the requirement of distributed computing applications. The distributed computing allows partitioning of application logic into units and spreading the unit over different................

Web services vs. CORBA and DCOM.
Web services use HTTP protocol for sending and receiving messages between the applications. The data encoding in web services is based on XML...............

Advantages of Web Services
Web Services are supported on a wide range of platforms. Web services may extend its interface and add new methods without affecting the clients as long as they still provide the old methods and parameters................

Explain in brief Web Service Standards.
Following are the standards used by web services: WSDL is used to create interface definition for a web services. It describes all about methods to the client, i.e. methods available in a web service, their parameters and return values..............

Define the specifications that help in the discovery of a web service.
DISCO, an abbreviation of discovery, is a file that groups together a list of related web services. A company that offers web services publishes a DISCO file on its server that has links of all the web services it provides..................

What are the data types supported by web Services?
.Net web services are built on XML-based standards for exchanging data. This means .NET web services can support only those data types that can be recognized by the XML schema standard...............

Testing and consuming a web service
.NET has a test web page that ASP.NET uses automatically when you request the URL of an .asmx file in a browser. This page uses reflection to read and show information about the web services, such as the names of the methods it provides...............

Difference between web service and remoting.net
Both .Net Web services and .Net Remoting are Microsoft solutions for distributed application. Before choosing one of them for application development, let’s identify the differences between these technologies....................